1. Field of the Invention
The present invention is directed to the preparation of mono-olefins by catalytic dehydrogenation of paraffinic hydrocarbons. In particular, the invention concerns certain improvements of industrial dehydrogenation processes for the preparation of mono-alkenes from corresponding alkanes having the same number of carbon atoms.
2. Description of the Related Art
The general reaction scheme in those processes can be expressed by the equilibrium reaction EQU C.sub.n H.sub.2n+2 .revreaction.C.sub.n H.sub.2n +H.sub.2 ( 1),
which is thermodynamically unfavourable towards formation of alkenes. Because of the high energy required to cleave a C--H bond, the reaction takes place at high temperature with extensive thermocracking and combustion of hydrocarbon feed. To minimize formation of by-products at desired production rates, industrial dehydrogenation processes employ catalysts, which allow the above equilibrium reaction to proceed at lower temperatures. Catalysts, conventionally used in the processes, are supported platinum catalysts, or catalysts comprising chromic oxide impregnated on activated alumina and platinum-tin-zinc aluminate in the form of cylindrical or spherical pellets.
An essential process variable in the catalytic dehydrogenation process is pressure. Since the process is thermodynamically limited, reduced pressure results in increased equilibrium conversion. Thus, a higher alkene concentration can be obtained when removing gaseous hydrogen from the process gas leaving reaction (1).
Oxidative hydrogen removal from dehydrogenated or oxygenated hydrocarbon feed in presence of a catalyst or a hydrogen retention agent is known in the art.
Removal of hydrogen by contact with a dehydrogenation catalyst being capable of adsorbing hydrogen is mentioned in EP 543,535. At the disclosed process, the feed is contacted with the catalyst above 500.degree. C. and hydrogen being formed during dehydrogenation is adsorbed on the catalyst. Catalysts, being able to adsorb hydrogen, are reducible metal oxides selected from Group IB, IIB and VIII of the Periodic Table. The hydrogen adsorbed on the catalyst is, subsequently, removed by applying heat, vacuum or by contact with an oxygen containing gas.
Dehydrogenation of hydrocarbons in separate beds of a dehydrogenation catalyst or in intermediate beds with a hydrogen selective oxidation catalyst is mentioned in U.S. Pat. No. 4,599,471 and U.S. Pat. No. 4,739,124. During the above processes, a dehydrogenated effluent stream from a bed of dehydrogenated catalyst is reheated and hydrogen is removed by passage through a subsequent bed of the hydrogen selective oxidation catalyst.
Use of alternating dehydrogenation and oxidation catalyst layers is further described in U.S. Pat. No. 3,855,330, U.S. Pat. No. 4,435,607 and U.S. Pat. No. 4,418,237. Formed hydrogen in the product gas is, thereby, removed by reaction with oxygen to steam in the presence of an oxidation catalyst.
In the known hydrogen removal processes, the employed catalysts are supported on highly porous inorganic support of alumina or ceria.
It has now been observed that catalyst activity and selectivity during catalytic hydrogen oxidation is limited by diffusion of reactants on the catalyst surface. Activity and selectivity of the catalysts are, thereby, strongly influenced by the number and size of surface pores. It has further been observed that even small changes in porosity of the catalyst surface result in considerable changes in activity and selectivity. Thereby, oxidation catalysts supported on highly porous support material show low selectivity at high temperatures.
In the dehydrogenation of alkanes, it is, however, required to carry out the process at high temperatures to provide practical dehydrogenation rates. At lower temperatures, the dehydrogenation equilibrium is, as mentioned before, unfavourable for the desired production of alkenes.
It has now been found that catalysts selected from the group of noble metals either in their pure metallic form or as alloys show improved catalytic activity and selectivity for the reaction of hydrogen with oxygen in a dehydrogenated carbonhydride process stream at high temperatures, when being used in their massive form.
Based on the above observations, it is believed that the low porosity of massive catalysts counteracts diffusion limitations and suppresses cracking and oxidation of carbonhydrates in such process gas.